Wireless communication systems are widely used to provide voice and data services for multiple users using a variety of access terminals such as cellular telephones, laptop computers and various multimedia devices. Such communications systems can encompass local area networks, such as IEEE 801.11 networks, cellular telephone and/or mobile broadband networks. The communication system can use one or more multiple access techniques, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA) and others. Mobile broadband networks can conform to a number of standards such as the main 2nd-Generation (2G) technology Global System for Mobile Communications (GSM), the main 3rd-Generation (3G) technology Universal Mobile Telecommunications System (UMTS) and the main 4th-Generation (4G) technology Long Term Evolution (LTE).
A wireless network may include a wireless device and a plurality of base stations. The wireless device may be a notebook computer, a mobile phone or a Personal Digital Assistant (PDA), a media player, a gaming device or the like. The base stations communicate with the wireless device over a plurality of wireless channels coupled between the wireless device and the base stations (e.g., a downlink channel from a base station to a wireless device). The wireless device may send back information, including channel information, to the base stations over a plurality of feedback channels (e.g., an uplink channel from the wireless device to the base station).
The wireless device may comprise a transmitter and a receiver coupled between an antenna and a baseband processor. A digital signal generated by the baseband processor may be mapped onto a constellation plot through which the digital signal may be converted into an in-phase signal and a quadrature signal. Both the in-phase signal and the quadrature signal are converted by local oscillators to in-phase and quadrature components having a carrier frequency. The in-phase and quadrature components are summed together before being sent to an input of a power amplifier. The power amplifier is employed to amplify the input signal to a desired power level suitable for radio communication.
The power amplifier is an important component in a wireless network. High efficiency and high linearity power amplifiers are desirable in order to improve the performance of the wireless network.
Envelope tracking is an effective technique to improve the efficiency of a power amplifier. The power consumption of a power amplifier is proportional to the bias voltage level of the power amplifier. A lowered bias voltage, however, may cause the nonlinearity of the power amplifier, which degrades the output signal of the power amplifier. If the bias voltage of the power amplifier is modulated such that the bias voltage follows the envelope of the output signal of the power amplifier, the linear operation of the power amplifier can be maintained. Such a modulated bias voltage helps to reduce the average value of the bias voltage, thereby reducing the average power consumption of the power amplifier.